Compositional band fluctuations were first considered for indium-gallium-nitride (InGaN) systems. It was found that the material properties of InGaN alloys change as the amount of indium in the alloy is increased. With the proper growth conditions, however, it was discovered that a material could be grown in which the indium did not incorporate uniformly throughout the InGaN layer (i.e., the material had areas of high and low concentrations of indium spread throughout). These compositional fluctuations, also known as localized inhomogeneities, result in carrier localization and lead to an enhancement in the radiative efficiency despite the high dislocation density. The discovery of the effects of the localized inhomogeneities enabled the development of commercially successful blue InGaN-based LEDs and laser diodes (LDs). It has been reported that the intense red-shifted photoluminescence (PL) peaks observed in InGaN alloys at room temperature result from the recombination of excitons localized at potential minima originating from large compositional fluctuations.
Similar localization effects were observed for aluminum-indium-gallium-nitride (AlInGaN) and aluminum-gallium-nitride (AlGaN) systems. The use of aluminum gallium nitride (AlxGa1-xN), as opposed to InAlGaN, is currently preferred as the base material for manufacturing ultraviolet (UV) light emitting diode (LED) devices for ultraviolet semiconductor optical sources operating at wavelengths between 260 to 360 nanometers (nm) due to its tunable band gap from 3.4 eV to 6.2 eV.
One approach discloses a semiconductor structure containing compositional fluctuations as well as a method for depositing group III-nitride films called molecular beam epitaxy (MBE). The structure comprises self-assembled nanometer-scale localized compositionally inhomogeneous regions. Within these regions, the luminescence occurs due to radiative recombination of carriers in the self-assembled nanometer-scale localized compositionally inhomogeneous regions having band-gap energies less than surrounding material. Further, another approach discloses self-assembled nanometer-scale localized compositionally inhomogeneous regions that include a fine scale facetted surface morphology or pits with diameters of about ten to one hundred nanometers. The approach also discloses the semiconductor device comprising of such semiconductor structures.
Group-III nitride based semiconductors are materials of choice for ultraviolet light emitting diodes, photomultipliers and photodiodes. Currently, wall plug operating efficiencies of deep ultraviolet light emitting devices reach only a few percent and a large effort is devoted to improving their efficiency.
Similar to InGaN-based semiconductor devices, carrier localization plays an important role in light emission from devices based on AlGaN semiconductor layers. Even though these materials are typically grown with a large number of threading dislocations and point defects, emission efficiency is higher than anticipated and radiative lifetimes obtained from photoluminescence studies are smaller than predicted by theory. This effect can be attributed to the carriers being isolated from nonradiative recombination centers due to localization in sites containing a smaller band gap than the surrounding semiconductor material.
FIG. 1 shows a schematic of compositional fluctuation according to the prior art. During the initial growth stage, adjacent small islands, from which the growth starts, coalesce into larger grains. As the islands enlarge, Ga adatoms, having a larger lateral mobility than Al adatoms, reach the island boundaries more rapidly. As a result, the Ga concentration in the coalescence regions is higher than in the center of the islands. The composition pattern, which is formed during the coalescence, is maintained as the growth proceeds vertically. As a result of the coalescence, the domain boundaries usually contain extended defects that form to accommodate the relative difference in crystal orientation among the islands. Even in layers with smooth surfaces containing elongated layer steps, screw/mixed dislocations occur due to the local compositional inhomogeneities.